1. Field of the Invention
This invention relates generally to display devices and more particularly to display devices in which only two states (on/off) or a limited number of discrete states are selectable for each picture element (pixel). More particularly, the present invention is to related to methods and apparatus for enhancing the gray shade rendering capability of a display device such as a liquid crystal display.
2. Description of the Related Art
Various devices for image display are available and their display capabilities vary greatly. For example, on a CRT display light is produced by three primary phosphors, red, green and blue (RGB), which are excited separately by the electron beam in the CRT. Through the application of a varying intensity electron beam to each of the adjoining red, green and blue phosphors (forming each pixel) a large gamut of colors and brightness levels can be produced. Color images are often represented as an array of pixels with the value of each pixel being represented by a 24-bit word, i.e. one 8-bit byte per color component. Each color component for each pixel can be represented by an intensity value ranging from 0 to 255. Color images (e.g. computer generated) for display on a CRT may include a large number of colors within this gamut or range.
In comparison to CRTs, liquid crystal displays (LCDs) have far less precision and may be limited to binary (on/off) with one bit per pixel or perhaps as many as four bits per pixel. LCDs have the capability to produce far fewer colors or shades of gray than can be represented by 8-bit pixel precision. LCDs are generally comprised of flat panels that are formed of a liquid crystal substance filling a clearance between two substrates. Images are displayed by controlling the orientation of the liquid crystal substance by an external signal to modulate the light, allowing it to pass through the panel or blocking it. Individual pixels are arranged in a matrix or array and are driven by a plurality of scanning electrodes and data electrodes. Generally, each pixel is controlled to be completely on or completely off (binary). In some devices intermediate gray levels can be depicted by applying incremental cell voltages that fall between full on and full off. However, there are practical limits on the generation and maintenance of such intermediate voltage levels. Color images can be produced in LCD displays through the use of color filter mosaics in registration with the individual pixel electrodes or using a white light separated by optics, such as dichroic mirrors, into red, green and blue components, which are modulated by the LCD panel.
In order to attempt to faithfully depict an image with a relatively high precision (e.g. 8-bits/pixel) on an LCD device, there must be an increase in the number of visually perceivable gray scale brightness levels. One approach is frame rate cycling or frame rate modulation in which a pixel is driven alternately on and off across multiple frame refreshes to produce a visual effect of the average intensity over the pattern cycle. For example, if a pixel is turned on during three frame refreshes and off for two it will appear to have a gray scale intensity of 3/5 for that refresh cycle. This approach can have drawbacks such as perceived flicker (where the image appears to be rapidly turned on and off) or swim (where the image appears to have artificial patterns that pass through it).
Various attempts have been made to reduce these visual drawbacks. For example, U.S. Pat. No. 5,642,133 to Scheffer et al. provides a number of gray levels for an LCD by modulating the amplitude or pulse height of the display column drive signals. However, such system requires multilevel drivers. U.S. Pat. No. 5,313,224 to Singhal et al. attempts to reduce flicker by spreading the phases of the modulating pixels across time and across the horizontal and vertical axes of the display. U.S. Pat. No. 4,921,334 to Akodes is one example which utilizes a combination of a multilevel driver and time multiplexing between successive frames. U.S. Pat. No. 5,608,649 to Garrett attempts to reduce flicker using an indiscernible pattern to cycle between on and off states. U.S. Pat. No. 5,389,948 to Liu varies the illumination of each pixel in accordance with the gray value of the corresponding pixel in the original image, the frame number, and the value of an element in a dither matrix. While these prior art methods may be helpful in reducing flicker or visual artifacts, they are limited in their ability to provide uniform dot patterns or a large number of gray shades.